LESSON Reteach Completing the Square

5-4 Completing the Square LESSON You can use the square root property to solve some quadratic equations. Square Root Property To solve x 2 a, take the...

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Name

Date

Class

Reteach

LESSON

5-4

Completing the Square

You can use the square root property to solve some quadratic equations. Square Root Property 2 To solve x  a, take the square root of both sides of the equation.

x2  a

Remember:



a x 2  

2 2 2  4, and  2   4.



x  a

2 Solve 4x  5  43. 2 4x  48

The coefficient of x 2 should be 1 to use the square root property.

Add 5 to both sides.

2 x  12

Divide both sides by 4.

 2



x  12

Take the square root of both sides.

x  12

Simplify.















Think: 12  4  3   4  3  2 3

x  23

2 Solve x  12x  36  50.

 x  6  2  50

Factor the perfect square trinomial.

 2

   x  6   50 Take the square root of both sides. 

x  6  50

Subtract 6 from both sides.



x  6   50

Simplify.





 



Think: 50  25  2  25  2  5 2



x  6  52 Solve each equation. 1. 3x  7  31

2. x  8x  16  18

2

3x 2 

2

2  x  4   18

24 x2  8



x  4   18





x  2 2

x  4  3 2

3. 6x 2  4  38

4. x 2  2x  1  10

6x 2  42 x2  7

x



x  1   10 



x   7

Copyright © by Holt, Rinehart and Winston. All rights reserved.

a207c05-4_rt.indd 30

 1  2  10

x  1   10

30

Holt Algebra 2

12/15/05 4:35:21 PM Process Black

Name

Date

Class

Reteach

LESSON

5-4

Completing the Square (continued)

You can use a process called completing the square to rewrite a quadratic of the form x 2  bx as a perfect square trinomial. To complete the square of x 2  bx, b 2. add __ 2

 

Think: Multiply the coefficient of x by 1 . Then square it. __ 2

   x  __2b 

2 b x  bx  __ 2

2

2

Complete the square: x 2  8x  ?. Identify b, the coefficient of x : b  8.

Step 1

   

Step 2 Step 3 Step 4

8 __b2   ___ 2 

b 2: Find __ 2 b 2: Add __ 2

2

x 2  8x  16

Factor:

x 2  8x  16   x  4  2

2

  4  2  16

Check:  x  4  2   x  4   x  4 

b as a factor. Use __ 2

 x 2  8x  16 ✓ Complete each square and factor. 2 5. x  9x  ?

6. x 2  4x  ?

9 b  __ b  9, so __ 2 2

2  b __

2

81 ___ 

4

b

4 , so __b  2

__b2 



2

2

4

81 ___ 4

x  9x  2

x 

9 __ 2



4

x  4x  2

2

x

7. x 2  10x  ?

8. x 2  3x  ?

9 x 2  3x  __ 4

x 2  10x  25 x

Copyright © by Holt, Rinehart and Winston. All rights reserved.

a207c05-4_rt.indd 31

 2 2



 5 2

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